Geothermal ground source/water source heat exchange systems typically include fluid-filled closed loops of tubing buried in the ground, or submerged in a body of water, which either absorb heat from or to reject heat into the naturally occurring geothermal mass and/or water surrounding the tubing. The tubing loop is extended to the surface and is then used to circulate the naturally warmed or cooled fluid to an interior air heat exchanger.
Common and older design geothermal water-source heating/cooling systems typically circulate, via a water pump, a fluid comprised of water, or water with anti-freeze, in plastic (typically polyethylene) underground geothermal tubing so as to transfer geothermal heat to or from the ground in a first heat exchange step. Via a second heat exchange step, a refrigerant heat pump system is utilized to transfer heat to or from the water. Finally, via a third heat exchange step, an interior air handler (comprised of finned tubing and a fan) is utilized to transfer heat to or from the refrigerant to heat or cool interior air space.
More recent geothermal heat exchange systems, known as direct exchange (or “DX”) systems, submerge the refrigerant transport lines below the surface, thereby eliminating one of the heat exchange steps noted above. The refrigerant transport lines in direct exchange systems are typically formed of copper and circulate a refrigerant fluid such as R-22, R-407C, R-410a, or the like. In a first heat exchange step, the refrigerant transport lines directly transfer geothermal heat to or from the sub-surface elements. Heat is transferred to or from an interior air space, typically using an interior air handler, in a second heat exchange step. Consequently, DX systems are generally more efficient than water-source systems because less heat exchange steps are required and because no water pump energy expenditure is necessary. Further, since copper is a better heat conductor than most plastics, and since the refrigerant fluid circulating within the copper tubing generally has a greater temperature differential with the surrounding ground than the water circulating within the plastic tubing, a direct exchange system generally requires less excavation and drilling, and therefore less installation costs, than a water-source system.
While most DX heat exchange designs are feasible, various improvements have been developed intended to enhance overall system operational efficiencies. Several such improvements, particularly in direct expansion/direct exchange geothermal heat pump systems, are taught in U.S. Pat. No. 5,623,986 to Wiggs; in U.S. Pat. No. 5,816,314 to Wiggs, et al.; in U.S. Pat. No. 5,946,928 to Wiggs; and in U.S. Pat. No. 6,615,601 B1 to Wiggs, the disclosures of which are incorporated herein by reference. Such disclosures encompass both horizontally and vertically oriented sub-surface heat geothermal heat exchanger.